Apparatus controlling accumulated electron charging of a nonconductive medium

ABSTRACT

Apparatus comprising in combination a first electron generator including focusing and scanning means for deflecting a primary electron beam to produce a bombarded scan pattern on a nonconductive medium, and control means including a second electron generator for generating a second electron beam of low energy electrons directed along an electron axis that is nonintersecting with the medium. The control means also including deflecting means to afford electron flooding of the medium by the low energy electrons and thereby cause secondary emission of electron from the medium, collecting means for collecting the secondary emission electrons to afford control of the accumulated charge on the medium, and light shield and trapping means to prevent illumination of the medium by the second electron generator. In the first embodiment, the medium is photon emissive and the apparatus includes a photomultiplier. In the second embodiment, the medium is sensitive to the bombarding electrons for producing a pattern thereon.

United States Patent [72] lnventor Robert S. Berglund Hudson, Wis.

[21] Appl. No. 142

[22] Filed Jan. 2, 1970 [45] Patented Oct. 12, 1971 [73] Assignee Minnesota Mining and Manufacturing Company Saint Paul, Minn.

[54] APPARATUS CONTROLLING ACCUMULATED ELECTRON CHARGING OF A NONCONDUCTIVE MEDIUM 5 Claims, 3 Drawing Figs.

[52] US. Cl 313/68,

178/66 TP,178/7.2 D, 313/80, 340/173 CR [51] Int. Cl Gllb 7/04,

G1 10 27/00,1-l01j 29/74 [50] Field of Search l78/6.6 A, 6.6 TP, 6.7 A, 6.7 R, 7.2 D; 340/173 CR; 313/68, 80

[56] References Cited UNITED STATES PATENTS 3,385,927 5/1968 Hamann 17817.5 D

3,491,236 1/1970 Newberry 340/173CR Primary Examiner-Stanley M. Urynowicz, Jr. Assistant Examiner-Howard W. Britton AttorneyKinney, Alexander, Sell, Steldt & Delahunt ABSTRACT: Apparatus comprising in combination a first electron generator including focusing and scanning means for deflecting a primary electron beam to produce a bombarded scan pattern on a nonconductive medium, and control means including a second electron generator for generating a second electron beam of low energy electrons directed along an electron axis that is nonintersecting with the medium. The control means also including deflecting means to afford electron flooding of the medium by the low energy electrons and thereby cause secondary emission of electron from the medium, collecting means for collecting the secondary emission electrons to afford control of the accumulated charge on the medium, and light shield and trapping means to prevent illumination of the medium by the second electron generator. In the first embodiment, the medium is photon emissive and the apparatus includes a photomultiplier. in the second embodiment, the medium is sensitive to the bombarding electrons for producing a pattern thereon.

APPARATUS CONTROLLING ACCUMULATED ELECTRON CHARGING OF A NONCONDUCTIVE MEDIUM BACKGROUND OF THE INVENTION An information reproducing system such as an Electron Beam Recorder commonly produces a picture, from an electrical signal, on an electron sensitive medium by direct electron beam bombardment of the medium. In such a system, a modulated electron beam scans, in response to scanning signals, and impinges electrons upon the electron sensitive medium located within an evacuatable chamber of the recorder. One problem commonly associated with an Electron Beam Recorder is that the bombarding electrons will charge the medium, which is substantially nonconductive, to a sufficient electrical potential which will deflect additional approaching electrons and cause the approaching electrons to impinge upon an area not intended to be bombarded. For example, in one application, an electron beam was to be used to write graphics within specified areas of a lattice formed on a nonconductive medium. The nonconductive medium obtained an accumulated charge potential sufficient to defocus and deflect the approaching electron beam and thus cause defocused graphics to be erroneously placed within unintended areas. One prior art solution to the accumulated charge problem has been the utilization of a nonconductive medium containing a conductive layer, such as a thin layer of vapor coated gold or aluminum, to control any charge accumulation. The use of a conductive larger is economically impractical due to the added expense of providing the additional conductive layer.

In an information retrieval system such as an Electron Beam Readout, the medium contains both a layer of previously imaged material, such as a photograph, and a layer of fluorescent material positioned adjacent to the imaged material. In such a system, an unmodulated beam of electrons is caused to scan the medium at a predetermined rate and within a predetermined pattern. The bombarding electrons strike the fluorescent material and cause corresponding fluorescent radiation of photon energy. The imaged material, including areas of opaqueness, transparency, and degrees of transmission ranging from the opaque to the transparent, differentially transmits the photon energy, generated by the bombarding electrons, in an inverse proportional relationship to the opaqueness of the image material in the area immediately being scanned by the electron beam. The differentially transmitted photon energy is then received by a photomultiplier which converts the transmitted light signal into an electrical signal which is indicative of the previously imaged material. One problem associated with an Electron Beam Readout is that the bombarding electrons will charge the medium, which is substantially nonconductive, to a sufficient potential which will defocus and deflect additional approaching electrons to impinge upon an area not intended to be bombarded. This defocusing and deflection will cause erroneous conversion of the imaged material into an electrical signal. One prior art solution to the charge accumulation problem has been the utilization of a medium containing a semitransparent conductive layer, such as a nonconductive thin layer of vapor coated gold or aluminum, to control any potential accumulated charge. Such a conductive layer could interfere with the normal optical exposure and/or development of the imaged layer. To avoid such exposure or development problems, the semitransparent conductive layer must then be applied after the optical exposure or the optical development of the imaged layer. This solution requires an additional coating process involving extra equipment, time, and expense.

If the problem of charge accumulation on a nonconductive medium could be effectively controlled, then an Electron Beam Recorder and an Electron Beam Readout could utilize a nonconductive medium, without a conductive layer, that would be less expensive because of the elimination of the additional conductive coating.

SUMMARY OF THE INVENTION The present invention relates to a new and useful electron bombarding system including control means within an evacuatable chamber for controlling the accumulated electrical potential on a nonconductive medium. The control means includes a second electron generating means for generating a second electron beam of rapidly diverging low energy electrons, apertured electrode means for defining an electron axis that is nonintersecting with the nonconductive medium, deflecting means for deflecting the second electron beam of low energy electron from the electron axis to afford flooding of the nonconductive medium with the low energy electrons and thereby cause secondary emission of more electrons from the nonconductive medium than received from the second electron beam, and collecting means located adjacent to the nonconductive medium for collecting the secondary emission electrons and thus afford control of the accumulated charge on the nonconductive medium.

The control means, therefore, eliminates the accumulated charge potential problem commonly associated with a high energy electron beam bombarding a substantially nonconductive medium. Thus, this system permits the utilization of a nonconductive medium in Electron Beam Recorders and Electron Beam Readouts without the accompanied spurious deflection of the primary electron beam by a charged medium.

Other uses and many of the attendant advantages of this invention will be readily understood and appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawing in which like numerals designate like parts throughout the figures and detailed description, and wherein:

FIG. 1 is a block diagram illustrating the first embodiment of the present invention;

FIG. 2 is a partial block diagram illustrating the second embodiment of the present invention; and

FIG. 3 is a partial cross-sectional view particularly illustrating the second electron generating means, the deflecting means, and the collecting means of the first embodiment.

In referring to the first embodiment illustrated in FIGS. 1 and 3, an improved Electron Beam Readout 10 comprises a housing 12 defining an evacuatable chamber 13 to contain a photon emissive nonconductive medium 14, first electron generating means 16 for generating a first electron beam 15 of high energy electrons and for directing the first electron beam 15 along a predetermined pat to bombard the nonconductive medium 14 positioned within the evacuatable chamber 13 whereby the bombarding high energy electrons will tend to accumulate an electrical potential on the nonconductive medium 14, scanning means 18 for deflecting the first electron beam 15 to produce a bombarded scan pattern directly on the nonconductive medium 14 in response to scanning signals 19, control means 24 within the evacuatable chamber 13 for controlling the accumulated electrical potential on the nonconductive medium 14, and a photomultiplier 70 located adjacent to the nonconductive medium 14 for receiving fluorescent radiation from the photon emissive nonconductive medium M.

The housing 12 includes a vacuum pump 11 and a corresponding power supply (not shown) to evacuate the gas within the evacuatable chamber 13.

The first electron generating means 16 includes an electron gun 17 powered by the electron gun power supply 23 and an electromagnetic focusing lens 20 to focus the electron beam 15 into a small cross-sectional area. A triode type electron gun 17 designed in accordance with the parameters set forth in a book entitled Electron Optics" by O. Klemperer published in 1953 by Cambridge University Press is illustrative of an electron gun 17 apparatus for use with this invention. Such an electron gun includes a wire filament, an anode, and a grid therebetween (none shown) to control the net number of electrons leaving the filament. The accelerated high energy electrons, having a typical energy value of 15 kilovolts, bypass the anode and are diametrically limited by the diameter of a limiting aperture within an apertured disk (not shown). The apertured disk defines the electrons into a primary or first electron beam and directs the first electron beam 15 along a predetermined path toward the nonconductive medium l4.

As the first electron beam 15 travels along the predetermined path, the first electron beam 15 passes through an electromagnetic focusing lens 20. The focusing lens functions to focus the electron beam 15 into a small cross-sectional area and thereby define the size of the area on the medium immediately bombarded by the first electron beam 15 as the first electron beam 15 scans the medium 14. The power supply used to operate the focusing lens is typically a constant current, low voltage power supply (not shown).

After leaving the focusing lens, the focused first electron beam 15 passes through a scanning means 18, illustrated as an electromagnetic deflection yoke 21. The deflection yoke 21 is adapted to deflect the first electron beam 15 in a predetermined scan pattern (such as a raster), in both a horizontal and vertical direction, on the medium 14, in response to scanning signals 19 provided by the deflection circuits 22. in place of the electromagnetic deflection yoke 21, one could employ electrostatic deflection plates to obtain a faster deflection of the beam, as those skilled in the art will appreciate.

The focused and deflected first electron beam 15 now impinges upon the nonconductive medium M of the first embodiment. The medium .14, as more clearly shown in FIG. 3, comprises a backing layer 28, an imaged layer 30, and a fluorescent layer 32. The backing layer 28, which is relatively translucent, provides structural support for the imaged and fluorescent layers 30, 32 the layer 28 could be selected from such materials as organic polymeric cellulose, polyesters, polyvinyl polymers or the like. The'fluorescent layer 32 includes scintillating and phosphoric materials for producing photons in response to bombarding electrons. The imaged layer could be selected from silver halide emulsions, diazo materials, thermographic materials, or the like. The imaged layer 30 comprises opaque areas 34, translucent areas 36 and areas 38 having degrees of transmission varying from translucency to opaque. The backing, imaged and fluorescent layers 28, 30, 32 could be positioned with the imaged layer 30 nearest to the electron gun 17 wherein the opaque and differentially transmissive imaged areas 34, 38 would prevent and restrict the bombarding high energy electrons from subsequently reaching the fluorescent layer 32. As illustrated in FIG. 3, the fluorescent layer 32 of this embodiment has been positioned to first receive the bombarding electrons wherein the generated photons 40 are prevented and differentially restricted from reaching the photomultiplier 70 by the opaque and differentially transmissive imaged areas 34, 38, respectively.

When the high energy electrons of the first electron beam 15 impinge against the fluorescent particles 41 within the fluorescent layer 32, the energized electrons will cause the fluorescent particles 41 to emit fluorescent radiation in the form of photons 40. As the fluorescent layer 32 is scanned by the bombarding electrons of the first electron beam, a corresponding generated light spot moves across the surface of the fluorescent layer 32. Whenever the moving light spot is located adjacent to an opaque area 34 of the imaged layer 30 the generated photons 40 will not be able to pass through the imaged layer 30 to reach the photomultiplier 70. Likewise, areas 38 having corresponding degrees of transmission will differentially transmit the photons 40 to the photomultiplier 70 at a quantity relationship that is proportional to the transmissive characteristic of the immediate imaged area adjacent to the generated light spot. Whenever the moving light spot is adjacent to a translucent or transparent area of the imaged layer a corresponding greater quantity of photons will be transmitted to the photomultiplier 70. The photomultiplier 70, powered by the photomultiplier power supply 71, in a known manner converts the differentially transmitted light into a corresponding electrical output signal 50.

When an insulator, such as a nonconductive medium 14, is bombarded by an electron beam, the bombarded surface will accumulate the impinging charges. If the nonconductive medium is flooded by a second electron beam 15 of low energy electrons, in this embodiment having energy values of approximately 100 volts, emission of secondary electrons 56 from the medium bombarded will be of a greater quantity than the quantity of low energy electrons received by the medium. This is also stated by saying that the secondary emission ratio of the medium 114 is greater than one. THe quantity of low energy electrons may be adjusted until the current of electrons 56 leaving the medium 14 is equal to the total current of electrons received by the medium from the first and second electron beams 35, 54. Thus, the potential the medium 14 will be, within a few volts of the potential of the secondary electron collector or collecting means 44 and in this embodiment the ring collector 45 is at ground potential. In referring to FIG. 3, the control means 34 within the evacuatable chamber 13 controls the accumulated electrical potential on the nonconductive medium 14 and includes a flood gun or second electron generating means 58 for generating a second electron 54 of rapidly diverging low energy beam electrons, an apertured electrode means 60 for defining an electron axis 62 that is nonintersecting with the nonconductive medium 14 and for directing the second electron beam 54 along the electron axis 62. The control means also includes deflecting means 64 including deflection plates 66 for deflecting the second electron beam 54 of afford flooding of the nonconductive medium 14 with the low energy electrons and thereby cause secondary emission of more electrons 56 from the nonconductive medium than received from the second electron beam 54. Collecting means 44 located adjacent to the nonconductive medium 141 collects the secondary emission electrons 56 from the nonconductive medium 14.

The second electron generating means 58, powered by the flood gun power supply 59 comprises an electron gun 74 includirig a hairpin filament 75, a grid 76, anode 77, focus electrode '78, and an apertured electrode means 60 for defining an electron axis 62 that is nonintersecting with the nonconductive medium 14 and for directing the second electron beam 54 along the electron axis 62. The amount of time for the medium surface potential to reach a predetermined equilibrium potential of the ring collector 45 is inversely proportional to the quantity of the low energy electrons within the beam 54. Typically, these time can be less than 0.01 second.

Since readout of a prerecorded medium typically depend upon very small variations in photon or light energy, any stray photon energy tends to decrease the signal to noise ratio in the electrical output signal from the photomultiplier. The control means 24, therefore, includes light shield means 68 located adjacent to the second generating means 58 for preventing light produced by the second electron generating means 58 from directly illuminating the medium 14 and light trapping means 72 located along the electron axis 62 of the second electron generating means 58 for trapping the illumination from the second electron generating means to thus prevent reflective illumination from illuminating the medium 14. The deflection plates 66 electrostatically deflect the second electron beam through an angle of approximately 70; the undeflected photons from the second electron generating means 58 are absorbed in the light trapping means 72.

The light trapping means 72 be constructed from aluminum honeycomb coated with carbon-black to absorb light while remaining conductive.

Any photon emission from the filament of the first electron gun 17 is effectively absorbed by the apertured anode plate (not shown) and does not effectively affect the medium 14 since the aperture within the apertured anode plate is extremely small, one to two mils in diameter.

In referring to the second embodiment illustrated in FIG. 2, an improved electron beam recorder comprises an evacuatable chamber 13 to contain an electron generating means for generating a modulated high energy first electron beam 82 and includes the control means 24 of the first embodiment for controlling the accumulated electrical potential on the electron sensitive nonconductive medium 81. The electron beam recorder 80, including the apparatus for generating, focusing, and scanning the modulated electron beam 82 is not shown in FIG. 2 but have been set forth in detail in US. Pat. No. 3,444,317, assigned to the same assignee as the present application, and is incorporated in this present second embodiment. in the second embodiment, the modulated electron beam 82 writes graphics on the electron sensitive nonconductive medium 81, such as conventional silver halide emulsion photographic film, and the control means 24 controls the charge accumulation of the nonconductive medium 81. Drive and takeup means 83 are diagrammatically shown in FIG. 2 and are sequentially operated as taught by U.S. Pat. No. 3,444,317.

While these embodiment of the invention have been shown and described, it will be appreciated that this is for the purpose of illustrations and that modifications may be made therein without departing from the spirit and the scope of the invention as set forth in the appended claims.

What is claimed is:

1. An electron bombarding system comprising in combination a. a housing defining an evacuatable chamber and adapted to contain a nonconductive medium;

b. first electron generating means within said evacuatable chamber for generating a first electron beam and for directing said first electron beam along a predetermined path to bombard the nonconductive medium positioned within said evacuatable chamber whereby the bombarding electrons will accumulate an electrical potential on the nonconductive medium;

c. scanning means for deflecting said first electron beam to produce a bombarded scan pattern directly on the nonconductive medium in response to scanning signals;

d. control means within said evacuatable chamber for controlling the accumulated electrical potential on the nonconductive medium, said control means including I. second electron generating means within said evacuatable chamber for generating a second electron beam of rapidly diverging low energy electrons, said second electron generating means including an apertured electrode means for defining an electron axis that is nonintersecting with the nonconductive medium and for directing said second electron beam along said electron axis;

2. deflecting means for deflecting said second electron beam of low energy electrons from said electron axis to afford flooding of the nonconductive medium with said low energy electrons and thereby cause secondary emission of more electrons from the nonconductive medium than received from said second electron beam; and

3. collecting means located adjacent to the nonconductive medium for collecting the secondary emission elec trons from the nonconductive medium to afford control of the accumulated charge of the nonconductive medium.

2. An electron bombarding system as defined in claim 1, wherein said control means includes light shield means located adjacent to said second generating means for preventing light produced by said second electron generating means from directly illuminating the medium. 3. An electron bombarding system as defined in claim 1, wherein said control means includes light trapping means located along said electron axis of said second electron generating means for trapping the illumination from said second electron generating means to thus prevent reflective illumination from illuminating the medium. 4. An electron bombarding system as defined in claim 2,

a. said system includes in comblnation the nonconductive medium, said medium including 1. an imaged layer including opaque and transparent areas,

2. a fluorescent layer adjacent to said imaged layer for producing fluorescent radiation upon bombardment by said first electron beam,

b. said system further comprising a photomultiplier located adjacent to said medium for receiving fluorescent radiation from said fluorescent layer when said first electron beam is impinging a nonopaque area of said imaged layer.

5. An electron bombarding system as defined in claim 2,

wherein said system includes in combination a nonconductive medium, and wherein said nonconductive medium is sensitive to the bombarding electrons of said first electron beam for producing a pattern thereon responsive to said scanning signals. 

1. An electron bombarding system comprising in combination a. a housing defining an evacuatable chamber and adapted to contain a nonconductive medium; b. first electron generating means within said evacuatable chamber for generating a first electron beam and for directing said first electron beam along a predetermined path to bombard the nonconductive medium positioned within said evacuatable chamber whereby the bombarding electrons wiLl accumulate an electrical potential on the nonconductive medium; c. scanning means for deflecting said first electron beam to produce a bombarded scan pattern directly on the nonconductive medium in response to scanning signals; d. control means within said evacuatable chamber for controlling the accumulated electrical potential on the nonconductive medium, said control means including
 1. second electron generating means within said evacuatable chamber for generating a second electron beam of rapidly diverging low energy electrons, said second electron generating means including an apertured electrode means for defining an electron axis that is nonintersecting with the nonconductive medium and for directing said second electron beam along said electron axis;
 2. deflecting means for deflecting said second electron beam of low energy electrons from said electron axis to afford flooding of the nonconductive medium with said low energy electrons and thereby cause secondary emission of more electrons from the nonconductive medium than received from said second electron beam; and
 3. collecting means located adjacent to the nonconductive medium for collecting the secondary emission electrons from the nonconductive medium to afford control of the accumulated charge of the nonconductive medium.
 2. deflecting means for deflecting said second electron beam of low energy electrons from said electron axis to afford flooding of the nonconductive medium with said low energy electrons and thereby cause secondary emission of more electrons from the nonconductive medium than received from said second electron beam; and
 2. An electron bombarding system as defined in claim 1, wherein said control means includes light shield means located adjacent to said second generating means for preventing light produced by said second electron generating means from directly illuminating the medium.
 2. a fluorescent layer adjacent to said imaged layer for producing fluorescent radiation upon bombardment by said first electron beam, b. said system further comprising a photomultiplier located adjacent to said medium for receiving fluorescent radiation from said fluorescent layer when said first electron beam is impinging a nonopaque area of said imaged layer.
 3. An electron bombarding system as defined in claim 1, wherein said control means includes light trapping means located along said electron axis of said second electron generating means for trapping the illumination from said second electron generating means to thus prevent reflective illumination from illuminating the medium.
 3. collecting means located adjacent to the nonconductive medium for collecting the secondary emission electrons from the nonconductive medium to afford control of the accumulated charge of the nonconductive medium.
 4. An electron bombarding system as defined in claim 2, wherein a. said system includes in combination the nonconductive medium, said medium including
 5. An electron bombarding system as defined in claim 2, wherein said system includes in combination a nonconductive medium, and wherein said nonconductive medium is sensitive to the bombarding electrons of said first electron beam for producing a pattern thereon responsive to said scanning signals. 